1. Field of the Invention
The present invention relates to a magnetoresistive element having a structure in which a current is supplied perpendicularly to the plane of the element, as well as a magnetic head, a magnetic recording apparatus and a magnetic memory which use the magnetoresistive element.
2. Description of the Related Art
The performance of magnetic devices has drastically been improved by discovery of a giant magnetoresistive effect (GMR) in a stacked structure of magnetic films. In particular, a spin-valve film (SV film) has a structure easily applicable to a magnetic device to enable the GMR effect to be effectively produced. Consequently, the spin-valve film has brought about marked technical improvement to magnetic heads and magnetic devices such as MRAMs (Magnetic Random Access Memories).
The “spin-valve film” is a stacked film having a structure in which a nonmagnetic metal spacer layer is sandwiched between two ferromagnetic layers. In the spin-valve film, the magnetization of one ferromagnetic layer (referred to as a “pinned layer” or “magnetization pinned layer”) is pinned by an antiferromagnetic layer or the like, whereas the magnetization of the other ferromagnetic layer (referred to as a “free layer” or “magnetization free layer”) is made rotatable in accordance with an external field (for example, a media field). In the spin-valve film, a giant magnetoresistance change can be produced by varying the relative angle between the magnetization directions of the pinned layer and free layer.
Conventional spin-valve films are CIP (Current In Plane)-GMR elements in which a sense current is supplied parallel to the plane of the element. In recent years, much attention has been paid to CPP (Current Perpendicular to the Plane)-GMR elements (referred to as “CPP elements” hereinafter) in which a sense current is supplied substantially perpendicular to the plane of the element.
The CPP element is advantageously applicable when there is a tendency to increasingly reduce the sizes of magnetic devices. However, CPP elements using a spacer layer consisting of a nonmagnetic metal provide a very low resistance change. It is thus difficult to obtain high output signals from such a CPP element.
A new type of CPP element has been proposed which uses a spacer layer comprising an insulating layer in which nano-scaled current paths (current confined paths) consisting of a nonmagnetic material punching through the insulating layer are formed. Such a CPP element exhibits a current confined path (CCP) effect (referred to as a CCP-CPP element hereinafter) and provides high output signals than a simple CPP element using a nonmagnetic metal spacer layer. However, if the CCP-CPP element were applied to a magnetic head adapted for high density recording, the MR ratio thereof might still be insufficient.
An element that might realize an MR ratio high enough to adapt a high recording density has been proposed which has a spacer layer in which current confined paths in an oxide layer are formed of a metal magnetic material and utilizes a ballistic magnetoresistive (BMR) effect (referred to as a BMR element hereinafter). See, for example, Jpn. Pat. Appln. Publication No. 2003-204095.
However, various problems are expected to result from the formation of a fine structure that realizes ballistic conduction in an oxide thin film. For example, the current paths must be reduced to a width of at most 1 nm in order to realize ballistic conduction, resulting in high resistance. For high density recording, an increase in resistance is not preferable because it adversely affects high frequency responses. It is possible to reduce the resistance by forming a large number of fine metal paths to establish a parallel conductor state. However, at a recording density of 500 Gbpsi or more, since the element size must be at most 60 nm square, it is very difficult to fabricate a structure having a large number of fine metal paths, in such a small element region.